1. Field of the Invention
The present invention relates to a linear oscillatory motion device which converts a constant rotational motion into a linear oscillatory motion and which is particularly useful for an optical card read/write apparatus and, more particularly, to such a linear oscillatory motion device capable of producing a linear oscillatory motion with a desirable speed variation.
2. Description of the Background Art
One conventional method of obtaining a linear oscillatory motion is by repeatedly reversing the direction of rotation of a motor and converting such rotations into a linear oscillatory motion by means of belt or rack, and pinion. However, such a conventional manner requires a fairly powerful motor in order for the motor to be able to carry out a rapid start up and quick reversing.
On the other hand, a linear oscillatory motion can be obtained from a continuous rotation of a motor in one direction by using a crank mechanism. An example of such a crank mechanism for an optical card read/write apparatus is shown in FIG. 1, where a crank mechanism comprising a crank arm 101 and a shaft 103 produces a linear oscillatory motion of a optical card 105 attached on a free end of the shaft 103, out of a rotational motion of a motor 107 driving the crank arm 101. Now, let the length of the crank arm 101 be R, the length of the shaft 103 be L, and a stroke of the optical card 105 be S, as shown in FIG. 1. Then, when the motor 107 drives the crank arm 101 at a constant angular velocity of .omega., a stroke S of the optical card 105 is given by the expression: ##EQU1##
The linear oscillatory motion produced by such a crank mechanism can be made to possesses various speeds by changing the length of the crank arm and the length of the shaft, but there are some limits in making this motion to conform with a desired speed variation. In particular, it is rather difficult for such a crank mechanism to produce an accurate constant speed linear oscillatory motion required by a optical card read/write apparatus.
One possible improvement on such a crank mechanism for the sake of producing an accurate constant speed linear oscillatory motion is to provide feedbacks of the crank speed, crank arm angle and phase, to the motor in order to perform an electrical rotational motion control according to comparison between such feedbacks and the angular velocity of the motor. However, this type of motion control calls for a large control device and, in addition, a moment of the motor and electric acceleration and deceleration control must meet severe requirements.
As a solution to this situation, the present applicant has proposed a linear oscillatory motion device using a crank mechanism capable of producing a constant acceleration in the accelerating and decelerating period of the linear oscillatory motion, as shown in FIG. 2, which is disclosed in Japanese patent laying open No. S62-127559.
In this device, the acceleration is changed abruptly as a step function at transitions between the accelerating or decelerating periods and the constant speed periods, as shown in FIG. 3. Consequently, an inertial force due to the mass of the driven object also makes an abrupt change. This causes an abrupt change in a loading torque of the motor, and in the revering period of the linear oscillatory motion the inertial force due to the mass of the driven object appears as an opposing force. As a result, harmful vibrations disturbing the smooth motion of the object arises, as shown in FIG. 4, which in turn generate annoying noises accompanying the motion.
Thus, conventionally, although it is not impossible to obtain a linear oscillatory motion from a continuous rotation of a motor in one direction with a desired speed variation, such a linear oscillatory motion is accompanied by harmful vibrations disturbing the smooth motion and annoying noises.